Demembranated muscle fibers catalyze a more rapid exchange between phosphate and adenosine triphosphate than actomyosin subfragment 1.

The rate of ATP in equilibrium with Pi exchange, that is, the incorporation of medium Pi into ATP during the net hydrolysis of ATP, has been measured for rabbit psoas muscle fibers, myofibrils, and actomyosin subfragment 1 (acto-S1). The maximum exchange rate in fibers at saturating [Pi] is 0.04 s-1 per myosin head at 8 degrees C, pH 7, and an ionic strength of 0.2 M. The dependence of the rate on Pi concentration can be approximated by a hyperbola with an apparent dissociation constant (Km) of 3 mM. Myofibrils catalyze ATP in equilibrium with Pi exchange with a similar Km but at a slightly lower rate. In contrast, the soluble acto-S1 system, in which ATP hydrolysis is not coupled to tension generation, catalyzes exchange at a rate 500 times lower than that of fibers at low Pi concentration, and the Km for Pi is greater than 50 mM. The difference between the ATP in equilibrium with Pi exchange of fibers and of acto-S1 is discussed in terms of a model in which Pi binds to a force-generating state AM'-ADP and, due to mechanical constraint, the average free energy of this state is higher in the fiber than in acto-S1.